164 M. E. Bevrich on Goniatiies. 



ing which they are only visible on the easts^ being completely 

 filled up by the shell. The shell is very delicately plicated on 

 the outer whorls ; the folds ai'e but little raised_, and leave 

 very indistinct traces an the casts. They become certainly 

 stronger on the inner whorls, without however being much 

 elevated. The direction of the folds changes as described 

 above, but the change commences further back, and we must 

 retrograde at least one entire whorl to perceive it. The A. 

 Diadema, as here represented, appears to differ totally from 

 A. Listeri in the form and structure of the shell, yet we ought 

 not to consider these ammonites as two different species, if 

 we w^ould rigorously follow tb-e principle of uniting all those 

 w^hich are connected by a continuous passage. In fact all 

 possible varieties of passage may be observed in these two 

 ammonites ; they are determined by the variations which 

 arise in the increase in height and the increase in breadth, 

 characters of great constancy in the Goniatites, but which 

 here vary between very distant limits. The increase in height 

 for ^. Listeri is estimated at 0*72; it is a very gradual increase. 

 For A, Diadema it is 0*52 : these are the two extremes. The 

 increase in breadth remains nearly equal in both. The pas- 

 sage of the form of A. Listeri into A. Diadema is influenced 

 thus : by following the continually rapid increase in height 

 the breadth of the back diminishes, and in consequence the 

 ridge, which is perceived as forming the limit between the back 

 and side, appears less and less evident. According as the 

 back becomes more straight, and the sides which begin to ap- 

 pear become larger, the umbilicus is narrowed and the plica- 

 tions of the shell become at the same time more numerous. 



18. Ammonites atratus, Gold. PI. II. fig. 8. a, b. 



The dorsal lobe is a little wider than deep, the middle ele- 

 vation reaches half the height of the lobe and is rounded, as 

 is also the dorsal saddle. In the lateral lobe the breadth and 

 depth are equal, the lateral saddle attains the same height as 

 the dorsal saddle, and declines but little towards the suture. 

 The increase in height is 0*4 ; the increase in breadth 0*5.5. 

 The inner whorls are completely enveloped without umbi- 

 licus. This ammonite is found with the two preceding at 



